Atomic Structure and Radioactivity
The atom and radioactivity
Atoms have a tiny central nucleus (protons + neutrons) surrounded by electrons in shells. The nucleus is about 1/10 000 the size of the atom, but holds almost all its mass. Some nuclei are unstable and break down, giving out radiation — this is radioactivity.
Isotopes
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Some isotopes are unstable (radioactive) and decay to become more stable.
Types of nuclear radiation
| Radiation | What it is | Penetration | Ionising power | Stopped by |
|---|---|---|---|---|
| Alpha (α) | 2 protons + 2 neutrons (a helium nucleus) | Low (few cm of air) | Very high | Paper / skin |
| Beta (β) | A fast-moving electron | Medium | Medium | A few mm of aluminium |
| Gamma (γ) | High-energy electromagnetic wave | High | Low | Thick lead / concrete |
Radiation is dangerous because it is ionising — it can damage cells and cause mutations. Alpha is most ionising but least penetrating; gamma is least ionising but most penetrating.
Half-life
The half-life of a radioactive isotope is the average time for half the unstable nuclei in a sample to decay (or for the count rate/activity to halve). Activity is measured in becquerels (Bq).
- After 1 half-life, half remains; after 2 half-lives, a quarter remains; after 3, an eighth, and so on.
Nuclear equations (decay)
- Alpha decay: the mass number falls by 4 and the atomic number falls by 2.
- Beta decay: a neutron turns into a proton (+ emitted electron), so the atomic number rises by 1 while the mass number stays the same.
Uses and dangers
- Uses: medical tracers and treatment (gamma), smoke alarms (alpha), sterilising equipment, checking pipes.
- Irradiation = being exposed to radiation from a source. Contamination = radioactive atoms getting onto or into an object/body (more dangerous long-term).
Worked example
A sample has an activity of 800 Bq and a half-life of 2 days. What is its activity after 6 days?
- 6 days = 3 half-lives. 800 → 400 → 200 → 100 Bq. ✓
Common mistakes
- Mixing up penetration: alpha is most ionising but least penetrating; gamma is the opposite.
- Getting alpha decay numbers wrong — mass −4, atomic number −2.
- Confusing irradiation (exposure) with contamination (radioactive material on/in you).
Exam tips
- Learn the penetration/stopping materials table.
- Practise half-life calculations by halving repeatedly.
- Know the changes to mass and atomic number in alpha and beta decay.
Key facts to remember
- Isotopes = same protons, different neutrons; unstable ones are radioactive.
- Alpha (helium nucleus, stopped by paper), beta (electron, stopped by aluminium), gamma (EM wave, stopped by lead) — decreasing ionising power, increasing penetration.
- Half-life = time for half the nuclei to decay; alpha decay: mass −4, atomic number −2; beta decay: atomic number +1.